1. Field of the Invention
The present invention relates to a shield structure with a heat sink layer for dissipating heat from a read sensor and, more particularly, to a shield structure which has a heat sink layer and a ferromagnetic layer wherein the heat sink layer includes a gold film and the ferromagnetic layer includes iron nitride (FeN) and nickel iron cobalt (NiFeCo) films.
2. Description of the Related Art
The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm above the rotating disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are embedded in a slider and have an air bearing surface (ABS) that is exposed for facing the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent the ABS causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk so that the read and write heads are positioned for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack) which are, in turn, sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic field in the pole pieces which causes flux across the gap at the ABS for the purpose of writing the aforementioned magnetic impression in tracks on the aforementioned rotating disk.
The read head includes a read sensor which is located between nonmagnetic nonconductive first and second read gap layers. The first and second read gap layers are located between ferromagnetic first and second shield layers. First and second leads are connected to the sensor for conducting a sense current IS therethrough and are further connected by additional leads to the processing circuitry. The magnetization of the sensor changes in response to signal fields from tracks on the rotating disk which causes a change in the resistance of the sensor. These resistance changes cause corresponding potential changes in the processing circuitry which are processed as playback signals.
The sense current IS is a major contributor to the generation of heat within the magnetic head. Another heat source is the write coil in the write head. Excessive heat generated within the magnetic head can degrade the magnetics of the ferromagnetic layers in the read sensor as well as causing pole tip protrusion. Pole tip protrusion is caused by heat expansion of the insulation stack which, in turn, causes an alumina overcoat layer to protrude beyond the ABS and destroy the head. Accordingly, the sense current IS and the write coil current are maintained at appropriate levels so that heat will not degrade performance of the head. It is known, however, that an increase in the sense current IS results in an increased detection of the signal fields from the rotating disk. An increase in the detected signal equates to increased storage capacity of the magnetic disk drive.
The first and second shield layers of the read head are the best candidates for reducing heat generated by the sensor. It is desirable that the materials employed for the first and second shield layers have the best heat conductivity possible for promoting heat dissipation. Constraints on the selection of shield materials, however, are acceptable hardness, sufficient magnetization (MS) to function as a shield and sufficient magnetic stability so that a magnetic moment of the shield layer fully returns to its easy axis orientation. Hardness is important because of a lapping process which laps the air bearing surface of the magnetic head. The ABS must be precisely lapped so that the sensor is established with a designed stripe height. Lapping is a grinding process which can smear soft materials across the ABS and cause shorting between the sensitive elements of the read sensor to the first and second shield layers. The shields are required to have high magnetization (MS) so that the shields will readily conduct signal fields from bits (magnetic impressions) on the rotating disk adjacent to the bit being read by the read head. In order to promote magnetic stability it is required that the shield material have a high uniaxial anisotropy (HK). The easy axis of the magnetic moment of each shield layer is parallel to the ABS and the surface planes of the shield layers. In a merged head, where the second shield layer also serves as a first pole piece layer, the write current rotates the magnetic moment of the second shield/first pole piece layer perpendicular to the ABS during the write function. If the magnetic moment does not return to the original parallel position to the ABS after relaxation of the write current the magnetic moment will magnetically influence the sensor in a different way which will alter the performance of the read head. Accordingly, it is important to consider the hardness of the materials and the magnetics of materials when selecting a material to improve the heat conductivity of the shield layers.
Typical materials employed for shield layers are Sendust (FeAlSi) and nickel iron (NiFe). Nickel iron (NiFe) is known to be a better heat sink than Sendust (FeAlSi), however, nickel iron (NiFe) has a tendency to smear across the ABS during the lapping operation. There is strong-felt need for providing shield layers which have heat conductivity, yet will provide the required hardness and magnetic properties mentioned hereinabove.
The present invention provides a shield stricture which includes a heat sink layer for dissipating heat and a ferromagnetic layer for satisfying the aforementioned magnetic properties of the shield layer. In a preferred embodiment, the heat sink layer includes a gold (Au) film. Gold (Au) is soft and will smear across the ABS unless properly supported. The gold (Au) film is sandwiched between first and second tantalum (Ta) films which are harder than gold and actually increase the hardness of the gold (Au) film itself. Gold (Au) has approximately four times the heat conductivity of nickel iron (NiFe). In a preferred embodiment the ferromagnetic layer includes an iron nitride (FeN) film and a nickel iron cobalt (NiFeCo) film. Iron nitride (FeN) is harder than Sendust (FeAlSi) or nickel iron (NiFe) which plays an important role in the invention which will be discussed hereinafter. Iron nitride (FeN) also has a high magnetization (MS) which is approximately twice the magnetization (MS) of nickel iron cobalt (NiFeCo). This is important so that the shield structure functions as a good magnetic shield for the sensor. On the other hand, nickel iron cobalt (NiFeCo) has a higher uniaxial anisotropy (HK) than iron nitride (FeN). This ensures that the magnetic moment of the shield layer will return to its original easy axis position parallel to the ABS after being rotated by an external magnetic field. While a single nickel iron cobalt (NiFeCo) film can be employed for a shield structure this would require that the nickel iron cobalt (NiFeCo) film be twice as thick as an iron nitride (FeN) film since the magnetization (MS) of the nickel iron cobalt (NiFeCo) film is one-half the magnetization (Ms) of an iron nitride (FeN) film. Accordingly, by employing the iron nitride (FeN) film between the heat sink layer and the nickel iron cobalt (NiFeCo) film the shield structure can be made thinner so that the heat sink layer is closer to the sensor for increased heat dissipation. An optional substitution for the nickel iron cobalt (NiFeCo) film is a nickel iron (NiFe) film and alloys thereof. While nickel iron (NiFe) and gold (Au) have a tendency to smear during lapping the tantalum (Ta) films promote the hardness of the gold (Au) film and the iron nitride (FeN) film in the ferromagnetic layer is harder than all of the films which provides support during the lapping process to reduce smearing of the softer films.
An object of the present invention is to provide a shield structure which has increased heat conductivity without sacrificing hardness and magnetic properties of the shield structure.
Another object is to provide one or more shield structures which enables an increase in the sense current IS without generating additional heat.
A further object is to provide a heat sink shield structure which provides high magnetization and high uniaxial anisotropy with a minimum thickness.
Other objects and advantages of the invention will become apparent upon reading the following description taken together with the accompanying drawings.